MOSCOW — Russian authorities opened a criminal case against a leader of the political opposition Wednesday on the basis of a television documentary broadcast by a Kremlin-friendly station that accused him of plotting mass riots.

After Sergei Udaltsov was brought in for questioning, liberal leaders quickly rose to his defense, even though he hails from the communist end of the political spectrum. They took his case as a sign that President Vladimir Putin intends to intensify efforts to suppress all dissident voices, and a protest movement that had appeared to be splintering rallied behind Udaltsov.

“Udaltsov and I are completely different,” Boris Nemtsov, a democratic leader from the 1990s, wrote on his blog. “He is a socialist, and I am a liberal. But right now this makes no difference. Regardless of our views, we must all stand up for Sergei.”

Protesters holding vastly dissimilar political views — nationalists, communists, socialists and free-market liberals — began joining street demonstrations last December objecting to Putin’s rule and demanding democratic reforms. Despite uneasiness over their differences, they drew tens of thousands of marchers during the winter and spring.

But their discomfort was openly visible during the last major march in September, which attracted far fewer demonstrators — the liberal Yabloko party pulled out, citing the presence of nationalists and left-wingers. On Wednesday, Yabloko’s leader, Sergei Mitrokhin, returned in spirit.

“We strongly protest against politically motivated police searches and call on the authorities to stop immediately,” Mitrokhin said in a statement. He said Udaltsov was a politician, not a criminal, and added, “The persecution of the political opposition should be stopped.”

A documentary this month on NTV, owned by Gazprom Media, an affiliate of the government-controlled energy conglomerate Gazprom, broadcast poor-quality footage taped by a hidden camera that purportedly showed Udalt­sov getting orders from a Georgian agent to set off riots in Russia. Georgia and Russia fought a war in 2008 and ever since have traded accusations of recriminatory plots.

“The film does not have any trace of video editing,” Vladimir Markin, a spokesman for the government investigative team, told the Interfax news agency Wednesday. “It was established that the voice heard in the ‘Anatomy of Protest 2’ film, including what was shot by hidden camera, is that of Udaltsov.”

Udaltsov, the 35-year-old head of the Left Front bloc, has been arrested frequently on protest-related charges and has staged numerous hunger strikes. He is a rousing speaker with a shaved head and serious demeanor and looks as if he was born holding a bullhorn. Although he has often been dismissed as a fringe leader, his commitment to dissent and diligence in organizing protests has earned him wider respect.

On Wednesday, he was led away by masked commandos after an hours-long search of his apartment. He was later released on his own recognizance after promising not to leave Moscow. Two of his associates are also under investigation. If charges are filed and Udaltsov is found guilty, he could face as much as 10 years in prison. He has denied the accusations.

Officials quickly lined up against him. “The dialogues the whole country saw and heard indicated much more serious crimes,” Irina Yarovaya, a member of the ruling United Russia party and head of the security committee in the lower house of parliament, told Interfax.

Alexander Torshin, a member of the upper house, suggested that the TV station probably had even more evidence. “I have a feeling that NTV journalists did not show everything they had at their disposal,” he said.

At the same time, Twitter accounts lit up with ridicule over the idea of using NTV as a source of evidence. Next, one tweet asserted, someone will be prosecuted because of evidence from a cartoon or comic book.

Putin, who returned to the presidency in May, has been encouraging increasingly harsh limits on the opposition. Fines for protest-related violations have been raised substantially, slander laws have been broadened and organizations that receive grants from abroad are required to register as foreign agents by Nov. 21. In addition, the U.S. Agency for International Development has been ordered out of the country for interfering in Russia’s internal affairs.

In another case announced Wednesday, prosecutors said they were investigating complaints that members of a coordinating council being set up by the opposition had embezzled donations.

“There is such danger,” said Lyudmila Alexeyeva, head of the Moscow Helsinki Group. “I am afraid that Sergei Udaltsov will only be the first target of the punitive machinery.”

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